Welding techniques include techniques performed by friction, such as friction stir welding (FSW). Such a technique is advantageous for assembling together metal parts for assembly that are plane or shaped, such as plates, sheets, or other analogous parts. The parts for assembly are held adjacent to each other and pressed against an anvil, by clamping or by some other analogous technique. The term “anvil” is used to designate any member suitable for forming a working surface or analogous member, with the parts being clamped thereto so that they are pressed against the anvil either directly or indirectly. By analogy, when welding parts that are shaped, the anvil may be itself be shaped to be complementary to the profile of the parts for assembly, so as to allow them to be pressed thereagainst. The parts are held pressed against the anvil by means of clamps, being located edge to edge in order to form the weld bead in the zone where the edges are adjacent to each other, corresponding to a weld zone. The parts may be put into abutment as a result of putting corresponding edge faces of the parts into contact with each other, or they may overlap, with a margin of one part lapped on a margin of the other part, in particular a part that includes a rabbet or the like for this purpose. A chuck has a shoulder and a welding pin provided with roughness in relief, which roughness is formed at its free end opposite from its end held by the chuck. The roughness in relief may be arranged as a thread, or as a section of polygonal shape for the free end of the welding pin. Placing the welding pin into contact with the adjacent edges of the parts and then organizing relative movement therebetween serves to form a weld bead and consequently to assemble the parts together. The weld bead is obtained by heating the parts under the effect of rubbing applied locally thereto by the welding pin in the weld zone. In addition, the shoulder presents a surface state that contributes to heating the parts for welding by friction. More particularly, friction with the welding pin and the shoulder leads to the material from which the parts are made heating up so as to obtain a desired pasty state for said material, with the materials of the two parts then mixing together. The two parts are then joined together by the continuous dynamic recrystallization of the material in the weld zone.
A problem arises in achieving accurate control over the temperature to which the parts are subjected during the welding operation. It is necessary to reach a determined welding temperature threshold in order to obtain a weld bead, but without that deforming the parts that are being assembled together. This problem becomes particularly difficult to solve when the parts for welding together are thin, and by way of indication they may present a thickness of the order of one millimeter or one-and-a-half millimeters, and/or for metal parts made of a material that presents poor thermal conductivity. Under such circumstances, friction welding induces localized heat generation very quickly because of the welding pin rubbing against the parts. This heat generation is particularly significant when the chuck is controlled in force, with the shoulder constituting a depth gauge by being pressed into contact with the parts. This localized and rapid temperature rise is likely to reach the melting temperature of the material from which the parts are made, and as a result, failing to obtain the desired pasty state, and more particularly not obtaining stable viscous spreading of the material constituting the parts as is required for the recrystallization step. It can thus be seen that a difficulty to overcome lies in obtaining temperature regulation in the weld zone of the material from which the parts are made, in particular when the metal parts are thin and even more so when they are made of a material presenting low thermal conductivity, or of respective materials that are different. Such temperature regulation is made more difficult to obtain when the welding pin is controlled in force with the shoulder that is associated therewith being pressed against the parts for welding.
In this field, it is conventional to implement a transient step for evaluating the dynamic characteristics specific to the welding pin prior to performing the welding operation. These characteristics relate in particular to the speed of rotation of the chuck and the speed of relative movement in translation between the welding pin and the parts along the weld zone. This transient step consists in placing the parts for welding together on the anvil and then in starting the weld bead over a required distance of the order of up to thirty centimeters in order to define as well as possible the ranges for the speeds of rotation of the chuck and for the speeds of relative movement in translation. Once the resulting welding conditions are satisfactory, the modeling of these dynamic characteristics is defined so as to obtain determined welding conditions, e.g. given the thickness of the parts for assembly, and the material from which they are made, the characteristics specific to the welding pin, and/or the environment of the welding zone. Nevertheless, when the parts are thin and/or of low thermal conductivity, or indeed of different thermal conductivities, the transient step is not sufficient to define in reliable and satisfactory manner the operations that are necessary for obtaining stable viscous spreading of the material in the weld zone. In addition, this transient step may be ineffective in the event of the parts for assembly being of respective thicknesses that vary along the weld bead that is to be formed. It is then necessary to reduce the speed of rotation and the speed of said relative movement in translation, thereby degrading the productivity of the welding operation.
Friction-welding techniques performed on parts of small thickness and/or low thermal conductivity require the welding pin to be controlled very precisely. When assembling together plane metal parts it is commonly preferred for the welding pin to work in an optimized force situation in order to increase the heating of the parts for assembly. Nevertheless, such working conditions are not suitable for parts of small thickness and/or low thermal conductivity because of the risk of the weld bead collapsing during welding. In addition, although controlling the welding pin in force can be satisfactory in terms of productivity and/or the advantage obtained by the shoulder acting as a depth gauge, it can also lead to increasing the risks of the welding pin breaking and of tending to cause the parts to stick to the anvil, which is to be avoided. As a result, for parts that are of small thickness and/or of low thermal conductivity, it is preferable in the end to perform the welding operation with the welding pin being controlled in position, i.e. using a chuck that does not have a shoulder since that tends to give rise to too great a temperature rise while forming the weld bead.
It can thus be seen that in this field there is a need to reconcile high rates of production throughput with rigorous control over the heating of the parts in order to obtain a weld bead that presents recrystallization characteristics, and thus mechanical qualities, that are satisfactory.
For example, document EP 1 048 390 (Fokker Aerostructures) proposes a friction-welding method that consists in optimizing the concentration of heat in the weld zone. In order to influence the distribution of heat produced by friction and to obtain such a concentration of heat in the weld zone, it is proposed to interpose massive cross-members between the clamps and the parts for assembly, said cross-members having a thermal conductivity coefficient that is analogous to that of the parts for assembly. An element of thermally insulating material is interposed between the parts for assembly and the anvil, and the welding pin is controlled in force to optimize the production of heat. The heat produced is typically concentrated in the immediate contact zone between the welding pin and the parts for assembly, by avoiding heat dispersion. The heat concentration capacity obtained in the welding zone enables production speeds to be considerably increased with minimum energy consumption. Such a solution for improving productivity is nevertheless not suitable for use with parts of small thickness because of the above-mentioned risks associated with the localized and rapid heat generation that is induced.
Proposals have also been made in U.S. Pat. No. 7,121,448 (Pazhayannur Ramanathan Subramanian et al.) to interpose a heater element between the parts and the anvil, in the welding zone and overlapping both parts, which heater element is formed by a plate extending along the weld bead that is to be formed. The heater element is incorporated in a frame, with the frame and the face of the anvil that supports the parts being flush. The frame also has cooling means, in particular ducts that convey a cooling liquid. The cooling means serve to control the temperature to which the heater element heats, and thus enables temperature to be raised in controlled manner, thereby avoiding localized zones of heat generation around the weld zone, and thus preserving the parts from deformation.
It should be observed that document U.S. Pat. No. 5,493,097 provides a welding method that does not make use of friction welding, and that does make use of an orifice facing the bottom faces of the parts for welding, the orifice being filled with a powder that encourages welding.